Negative active material for secondary battery and secondary battery using the same

ABSTRACT

Disclosed is a negative active material having a reduced volume change during charge/discharge. A fine crystalline region exists on a matrix of an alloy, and lithium is easily dispersed due to the fine crystalline region. In the negative active material, a ratio of the fine crystalline region is represented by an amorphization degree, and is optimized in designing a battery through a measurement of an expansion rate after 50 cycles. The negative active material is used for a secondary battery and includes an alloy formed by a chemical formula below: (a ratio of Ti to Fe is 1:1 and a ratio of Si:Ti or Si:Fe has a range of 5:1 to 9:1.)
 
Si— −   x Ti y Fe z Al u  
 
where x, y, z, and u are at %, x: 1−(y+z+u), y: 0.09 to 0.14, z: 0.09 to 0.14, and u: larger than 0.01 and less than 0.2.

CROSS REFERENCE TO PRIOR APPLICATIONS

This application is a nation stage application of a PCT internationalapplication No. PCT/KR2015/012993, filed on Dec. 1, 2015, which claimspriority to and the benefit of Korean Patent Application No.10-2015-0001838 filed in the Korean Intellectual Property Office on Jan.7, 2015, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a negative active material for asecondary battery and a second battery using the same.

BACKGROUND ART

A lithium battery in the related art uses a lithium metal as a negativeactive material, but when a lithium metal is used, a battery isshort-circuited by formation of dendrite to cause danger of explosion,so that a carbon-based material is widely used as a negative activematerial, instead of a lithium metal.

The carbon-based active material includes crystalline carbon, such asgraphite and synthetic graphite, and amorphous carbon, such as softcarbon and hard carbon. However, the amorphous carbon has a largecapacity, but has a problem in large irreversibility during acharge/discharge process. Graphite is representatively used as thecrystalline carbon, and has a theoretical limit capacity of 372 mAh/g,and may be used as a negative active material.

However, even though a theoretical capacity of the graphite or thecarbon-based active material is slightly large, the theoretical capacityis simply about 380 mAh/g, so that there is a problem in that theaforementioned negative electrode cannot be used when a large capacitylithium battery is future developed.

In order to solve the problem, research on a metal-based orintermetallic compound-based negative active material has been currentlyand actively conducted. For example, research on a lithium batteryutilizing metal, such as aluminum, germanium, silicon, tin, zinc, andlead, or semimetal as a negative active material has been conducted. Thematerial has a large capacity and a high energy density, and is capableof occluding and discharging larger lithium ions than the negativeactive material using the carbon-based material, so that it is possibleto manufacture a battery having a large capacity and a high energydensity. For example, it is known that pure silicon has a largetheoretical capacity of 4,017 mAh/g.

However, compared to the carbon-based material, the metal-based orintermetallic compound-based negative active material has a cyclecharacteristic degradation to be obstacles to commercialization. Thereason is that when the silicon is used as a negative active materialfor occluding and discharging lithium as it is, conductivity betweenactive materials may deteriorate due to a change in a volume during acharge/discharge process, or a negative active material is peeled from anegative current collector. That is, the silicon included in thenegative active material occludes lithium by charging and is expanded tohave a volume of about 300 to 400%, and when lithium is dischargedduring the discharge, mineral particles are contracted.

When the aforementioned charge/discharge cycle is repeated, electricinsulation may be incurred due to a crack of the negative activematerial, so that a lifespan of the lithium battery is sharplydecreased. Accordingly, the aforementioned metal-based negative activematerial has a problem to be used in the lithium battery.

In order to solve the aforementioned problem, research on a negativeactive material having a buffering effect against a volume change byusing particles having a nano size level as silicon particles or givingporosity to silicon is conducted.

Korean Patent Application Laid-Open No. 2004-0063802 relates to“Negative Active Material for Lithium Secondary Battery, Method ofManufacturing the Same, and Lithium Secondary Battery”, and adopts amethod of alloying silicon and another metal, such as nickel, and theneluting the metal, and Korean Patent Application Laid-Open No.2004-0082876 relates to “Method of Manufacturing Porous Silicon andNano-size Silicon Particle, and Application of Porous Silicon andNano-size Silicon Particle as Negative Electrode Material for LithiumSecondary Battery”, and discloses technology of mixing alkali metal oralkali earth metal in a powder state with silicon precursor, such assilicon dioxide, performing heat treatment on a mixture, and eluting themixture as acid.

The patent applications may improve an initial capacity maintenance rateby a buffering effect according to a porous structure, but simply useporous silicon particles having conductivity deterioration, so that whenthe particles do not have a nano size, conductivity between theparticles is degraded while manufacturing an electrode, thereby causinga problem of deterioration of initial efficiency or a capacitymaintenance characteristic.

DISCLOSURE Technical Problem

The present invention has been made in an effort to provide a negativeactive material for a lithium secondary battery, of which a change in avolume is small during charge/discharge, so that electric insulation isless incurred.

The present invention has also been made in an effort to provide anegative active material for a lithium secondary battery havingexcellent initial efficiency and an excellent capacity maintenancecharacteristic.

The present invention has also been made in an effort to provide anoptimized negative active material in consideration of an amorphizationdegree when designing a battery.

Technical Solution

An exemplary embodiment of the present invention provides a negativeactive material for a secondary battery, which is an alloy formed by achemical formula below, and in which a ratio of Ti to Fe in the negativeactive material for the secondary battery has 1:1 and a ratio of Si:Tior Si:Fe in the negative active material for the secondary battery has arange of 5:1 to 9:1.Si—⁻ _(x)Ti_(y)Fe_(z)Al_(u)(x, y, z, and u are at %, x: 1−(y+z+u), y: 0.09 to 0.14, z: 0.09 to0.14, and u: larger than 0.01 and less than 0.2)

An amorphization degree of a fine crystalline region on a matrix withinthe alloy may be 25% or more and an expansion rate of the negativeactive material after 50 cycles may have a range of 70 to 150%.

Al in the negative active material for the secondary battery may have arange of 5 to 19% based on atom (%) (at %).

Al in the negative active material for the secondary battery may have arange of 10 to 19% based on at (%).

Each of Ti and Fe in the negative active material for the secondarybattery may have a range of 9 to 12.5% based on at (%).

A discharge capacity of the negative active material for the secondarybattery after 50 cycles may be 90% or more compared to an initialdischarge capacity.

Efficiency of the negative active material for the secondary batteryafter 50 cycles may be 98% or more.

Another exemplary embodiment of the present invention provides asecondary battery, including: a negative electrode including a negativeactive material, a positive electrode; and an electrolyte, and thenegative active material is an alloy formed by a chemical formula below,and a ratio of Ti to Fe in the negative active material for thesecondary battery has 1:1, and a ratio of Si:Ti or Si:Fe in the negativeactive material for the secondary battery has a range of 5:1 to 9:1, andSi has a range of 60 to 70%, Ti has a range of 9 to 14%, Fe has a rangeof 9 to 14%, and Al has a range of 5 to 19% based on at (%).Si_(x)Ti_(y)Fe_(z)Al_(u)  Chemical Formula:(x, y, z, and u are at %, x: 1−(y+z+u), y: 0.09 to 0.14, z: 0.09 to0.14, and u: 0.05 to 0.19)

Advantageous Effects

According to the present invention, it is possible to obtain thenegative active material for a lithium secondary battery, which has asmall change in a volume during charge/discharge, so that electricinsulation is less incurred, and has excellent initial efficiency and anexcellent capacity maintenance characteristic.

Further, the present invention may provide a value of an amorphizationdegree of the optimized negative active material in designing a batterythrough a measurement of an expansion rate after 50 cycles.

Further, the present invention may provide the optimized negative activematerial in consideration of an amorphization degree when designing abattery.

DESCRIPTION OF DRAWINGS

FIGS. 1A, 1B, and 1C are tissue pictures of an expansion characteristicmeasured after 50 cycles for a negative active material according to theComparative Examples.

FIG. 2 is a tissue picture of an expansion characteristic measured after50 cycles for a negative active material according to the Examples ofthe present invention.

FIGS. 3A and 3B are diagrams illustrating measurement of anamorphization degree of the negative active material according to theExamples of the present invention.

BEST MODE

An exemplary embodiment of the present invention provides a negativeactive material for a secondary battery, which is an alloy formed by achemical formula below, and in which a ratio of Ti to Fe in the negativeactive material for the secondary battery has 1:1 and a ratio of Si:Tior Si:Fe in the negative active material for the secondary battery has arange of 5:1 to 9:1.Si—⁻ _(x)Ti_(y)Fe_(z)Al_(u)(x, y, z, and u are at %, x: 1−(y+z+u), y: 0.09 to 0.14, z: 0.09 to0.14, and u: larger than 0.01 and less than 0.2)

MODE FOR INVENTION

Other detailed matters of the exemplary embodiments are included in thedetailed description and the drawings.

Various advantages and features of the present disclosure and methodsaccomplishing thereof will become apparent from the following detaileddescription of exemplary embodiments with reference to the accompanyingdrawings. However, the present invention is not limited by the exemplaryembodiments disclosed below, but may be implemented in various forms.Throughout this specification and the claims that follow, when it isdescribed that an element is “coupled” to another element, the elementmay be “directly coupled” to the other element or “electrically coupled”to the other element through a third element. Further, an irrelevantpart to the present invention is omitted to clarify the description ofthe present invention, and like reference numerals designate likeelements throughout the specification.

Hereinafter, the present invention will be described in detail withreference to the accompanying drawings.

An exemplary embodiment of the present invention provides a negativeactive material for a secondary battery with an improved expansion rate,and a secondary battery including the same. Particularly, in theexemplary embodiment of the present invention, it is possible to obtaina negative active material for a secondary battery, in which anamorphization degree of a fine crystalline region on a matrix within analloy is 25% or more, in the negative active material.

In general, when a silicon-based negative active material is studied, itis measured how much an electrode plate thickness during a full chargeat the first cycle after a formation process is increased compared to aninitial electrode plate thickness (an electrode plate thickness beforean electrolyte is injected). That is, an expansion rate after one cycleis measured, and here, a change in volume of the negative activematerial is exhibited while the negative active material occludeslithium.

However, in the exemplary embodiment of the present invention, anexpansion rate after 50 cycles was measured by measuring a thickness ofan electrode plate after repeat of charge and discharge for 50 cycles,not one cycle, and comparing the measured thickness and an initialelectrode plate thickness. Through the measurement of the expansion rateafter 50 cycles, it is possible to monitor a change in volume accordingto occlusion and discharge of lithium and a generation degree of a SolidElectrolyte Interface or Interphase (SEI) layer which is piled while anelectrolyte is decomposed by a side reaction generated on a surface ofan active material.

When a characteristic of a silicon-based negative active material isevaluated by fabricating a coin half cell, a lithium metal electrodeused as a counter electrode generally begins to deteriorate after 50cycles, thereby influencing a result. Accordingly, in the exemplaryembodiment of the present invention, a change in a thickness of anelectrode plate is measured by deconstructing a coin cell after theevaluation of a lifespan for 50 cycles, so that not only expansion of aninitial electrode plate by simple lithium occlusion but also expansionof an electrode plate according to growth of a side reaction layer forsubsequent 50 cycles are taken as indexes of an evaluation ofperformance of a negative active material. Accordingly, in the exemplaryembodiment of the present invention, it was found that a change in anexpansion rate after 50 cycles is a considerably meaningful evaluationindex, and thus it was possible to derive an optimum component range.

Commonly, for graphite, a very stable SEI layer is generated at aninitial formation charging stage, and a change in volume of an electrodeplate is 20% or less after an initial charging stage, so that the SEIlayer tends to be maintained at the initial charging stage as it iswithout a distinct change. However, in a silicon-based negative activematerial, a change in volume of an electrode plate is large, so that aphenomenon, in which a surface of a new active material is exposed to anelectrolyte while an SEI layer, which is initially generated on asurface of the active material, is separated when the active material iscontracted, and a new SEI layer is generated on the surface during nextexpansion, is repeated, so that a side reaction layer, which is a verythick SEI layer, is developed.

The side reaction layer piled on the surface of the active materialserves as a resistor and disturbs a movement of lithium within thesecondary battery, and the electrolyte is consumed for forming the sidereaction layer, thereby causing a problem in that a lifespan of abattery is decreased. Further, an increase in a thickness of theelectrode plate according to the development of the side reaction layercauses physical deformation of a jelly-roll of the battery, and acurrent is concentrated to a partial area of the electrode plate,thereby causing a phenomenon in that the battery is rapidly degraded.

For a silicon alloy material in the related art, there is a case where amatrix exists within the active material as it is while charge anddischarge are repeated, and only a silicon part is contracted andexpanded, so that a crack is generated between the matrix and silicon.In this case, it is found that an electrolyte permeates the crack and aside reaction layer of the electrolyte is generated within the activematerial, so that the active material is dispersed, and in this case, asharp increase in a thickness of the electrode plate after 50 cycles isobserved.

This phenomenon cannot be found during the measurement of the thicknessof the electrode plate after one cycle, and implies that even though thesilicon-alloy material has an excellent initial expansion rate, when thesilicon alloy material is actually applied to a battery, the siliconalloy material may cause various problems, such as an increase ininternal resistance within the battery and depletion of the electrolyte.Accordingly, the expansion of the electrode plate after 50 cyclessuggested in the present exemplary embodiment is a very usefulevaluation index for evaluating expansion, contraction, and sidereaction phenomena of the active material when developing asilicon-based negative active material.

In the exemplary embodiment of the present invention, a size of anexpansion rate after 50 cycles is investigated according to acomposition of a metal compound for a negative active material used inthe exemplary embodiment of the present invention to derive a range ofan optimum expansion rate according to a change in composition.

In the meantime, in the exemplary embodiment of the present invention, afine crystalline region exists on a matrix of an alloy, thereby makinglithium be more easily dispersed. Further, a rate of the existence ofthe fine crystalline region may be represented by an amorphizationdegree, and the amorphous region is formed on the matrix, so that avolume expansion while charging the secondary battery may be restricted.

The present invention is characterized in that an amorphization degreeof the fine crystalline region on the matrix is 25% or more. When theamorphization degree is formed within the range, lithium is considerablyeasily dispersed. Further, it can be seen that an expansion rate after50 cycles is also excellently exhibited within the aforementioned rangeof the amorphization degree, and thus, when the aforementioned materialis used as a negative active material, volume expansion is restrictedduring charging.

In the exemplary embodiment of the present invention, an amorphizationdegree may be 25% or more when an XRD pattern rotation angle of an alloy2θ is in a range of 20° to 100°. Within the range of the amorphizationdegree, the volume expansion is restricted, so that electric insulationis generated well.

A calculation of an amorphization degree used in the present inventionis as follows, and an amorphization degree may be calculated accordingto the illustration of FIG. 3.Amorphization degree (%)=((entire area−crystallization area))÷entirearea)

In the exemplary embodiment of the present invention, a largeamorphization degree means that there are many fine crystalline regions,and thus, lithium ions are accumulated by a buffering effect in the finecrystalline region during charging, so that it is possible to obtain aneffect in that a main factor of a volume expansion is restricted.

Further, in the exemplary embodiment of the present invention, anexpansion rate after 50 cycles has a range of 70 to 150%, and a negativeactive material for a secondary battery formed by an equation below isprovided.Si_(x)Ti_(y)Fe_(z)Al_(u)  (1)

(x, y, z, and u are atom % (at %), x: 1−(y+z+u), y: 0.09 to 0.14, z:0.09 to 0.14, and u: larger than 0.01 and less than 0.2)

In the present exemplary embodiment, Si has a range of 60 to 70% and Tiand Fe have a range of 9 to 14% based on at %. However, Al has a rangelarger than 1% and less than 20%, but, preferably, a range of 5 to 19%.

Ti and Fe included in the alloy is bonded to Si to form an intermetalliccompound of Si₂TiFe. Accordingly, when a content of each of Ti and Fe is14 at %, 28 at % or more of Si is consumed for forming the intermetalliccompound, so that a capacity of Si per g of the active material isdecreased, and in this case, in order to obtain Si with a capacity of1,000 mAh/g or more, the content of Si inserted needs to be considerablyincreased.

In general, when a large amount of Si that is a semimetal is contained,viscosity of a molten metal is high during melting, and thus rapidsolidification workability becomes poor, so that the content of Si ismaintained within a range of 70% as possible as it can, and thus,preferably, the contents of Ti and Fe do not exceed 14%. In theexemplary embodiment of the present invention, it was derived that it ispreferable to decrease the contents of Ti and Fe to 14% or less during aprocess of drawing an optimum alloy component in relation to anexpansion rate.

Further, Al may have a range larger than 1% and less than 20% based onat %. When about 1% of Al is included, expansion of the active materialis severely incurred after 50 cycles, and the active material isdispersed, so that about 1% of Al is not preferable. Further, when 20%of Al is included, a discharge capacity is decreased by a change in avolume fraction of Si:matrix, so that 20% of Al is not preferable. Inthe exemplary embodiment of the present invention, it was derived thatwhen Al has a range of 5 to 19% based on at %, the active material has arange of the most preferable expansion rate, and it could be seen that adischarge capacity is not decreased within the range of 5 to 19%. Mostpreferably, Al is 10 to 19%, and it is possible to obtain the range of amost preferable 50 cycle expansion rate, and further a dischargecapacity is not decreased.

Further, a method of preparing the negative active material of thepresent invention is not particularly limited, and for example, variousfine powder preparing methods (a gas atomizer method, a centrifugal gasatomizer method, a plasma atomizer method, a rotating electrode method,and a mechanical alloying method) publicly known in the art may be usedas the method. In the present invention, it is possible to prepare anactive material by applying, for example, a single roll rapidsolidification method of mixing Si and a component forming the matrix,melting a mixture by an arc melting method, and the like, and thenspraying the melt to a rotating copper roll. However, a method appliedin the present invention is not limited to the aforementioned method,and as long as a method may sufficiently obtain a rapid solidificationspeed, other than the single roll rapid solidification method, theaforementioned suggested fine powder preparing method (the gas atomizermethod, the centrifugal gas atomizer method, the plasma atomizer method,the rotating electrode method, and the mechanical alloying method) maybe used.

Further, it is possible to manufacture a secondary battery by using thenegative active material according to the exemplary embodiment of thepresent invention, and the secondary battery may include a lithiatedintercalation compound as a positive electrode, and further, inorganicsulfur (S₈, elemental sulfur) and a sulfur compound may be used, andexamples of the sulfur compound include Li₂S_(n) (n≥1), Li₂S_(n) (n≥1)melt in catholyte, and an organic sulfur compound or a carbon-sulfurpolymer ((C₂S_(f))_(n), f=2.5 to 50, n≥2).

Further, the kind of electrolyte included in the secondary battery ofthe present invention is not particularly limited, and a general meanspublicly known in the art is adoptable. In one example of the presentinvention, the electrolyte may include a nonaqueous organic solvent andlithium salt. The lithium salt is melt in an organic solvent, so thatthe lithium salt may serve as a lithium ion supply source within thebattery, and facilitate a movement of lithium ions between the positiveelectrode and a negative electrode. Examples of the lithium salt usablein the present invention include one kind or two or more kinds of LiPF₆,LiBF₄, LiSbF₆, LiAsF₆, LiCF₃SO₃, LiN(CF₃SO₂)₃, Li(CF₃SO₂)₂N, LiC₄F₉SO₃,LiClO₄, LiAlO₄, LiAlCl₄, LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) (here,x and y are natural numbers), LiCl, LiI, and lithium bisoxalate borateas supporting electrolytic salt. A concentration of lithium salt in theelectrolyte may be changed depending on a usage, and generally is in arange of 0.1 M to 2.0 M.

Further, the organic solvent serves as a medium for making ionsinvolving in an electrochemical reaction of the battery move, and anexample thereof includes one or more of benzene, toluene, fluorobenzene,1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene,1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene,1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene,1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene,1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene,1,2,3-triiodobenzene, 1,2,4-triiodobenzene, fluorotoluene,1,2-difluorotoluene, 1,3-difluorotoluene, 1,4-difluorotoluene,1,2,3-trifluorotoluene, 1,2,4-trifluorotoluene, chlorotoluene,1,2-dichlorotoluene, 1,3-dichlorotoluene, 1,4-dichlorotoluene,1,2,3-trichlorotoluene, 1,2,4-trichlorotoluene, iodotoluene,1,2-diiodotoluene, 1,3-diiodotoluene, 1,4-diiodotoluene,1,2,3-triiodotoluene, 1,2,4-triiodotoluene, R—CN (here, R is ahydrocarbon group having a linear, branched, or ring structure with 2 to50 carbon atoms, and the hydrocarbon group may include double bonding,aromatic ring, or ether bonding), dimethylformamide, dimethylacetate,xylene, cyclohexane, tetrahydrofuran, 2-methyltetrahydrofuran,cyclohexanone, ethanol, isopropyl alcohol, dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, methyl propyl carbonate, propylenecarbonate, methyl propionate, ethyl propionate, methyl acetate, ethylacetate, propyl acetate, dimethoxyethane, 1,3-dioxolane, diglyme,tetraglyme, ethylene carbonate, propylene carbonate, γ-butyrolactone,sulfolane, valerolactone, decanolide, and mevalerolactone, but is notlimited thereto.

The secondary battery of the present invention may further include atypical element, such as a separator, a can, a battery case, or agasket, in addition to the aforementioned elements, and a particularkind of the element is not particularly limited. Further, the secondarybattery of the present invention may include the aforementioned elementsand be manufactured by a method and in a shape general in the art. Anexample of the shape of the secondary battery of the present inventionincludes a cylindrical shape, a horn shape, a coin shape, or a pouchshape, but the shape thereof is not limited thereto.

Example 1

The present invention will be described in more detail through theExamples.

In the present Example, Si has a range of 60 to 70% based on at %, andTi and Fe have a range of 9 to 14% based on at %. In the meantime, theAl has a range larger than 1% and less than 20%, and preferably, a rangeof 5 to 19%. Most preferably, the Al has the range of 10 to 19%.

Table 1 below is a table representing a composition range of theExamples of the present invention and the Comparative Examples. In themeantime, Table 2 below relates to an evaluation of a negative activematerial based on the compositions of Table 1, and particularly,represents a 1CY-charge/discharge quantity, 1CY-efficiency, a1CY-electrode plate capacity, a 50CY-discharge capacity,50CY-efficiency, a 50CY-capacity maintenance rate, a 50CY-expansionrate, and an amorphization degree (%) of the Examples and theComparative Examples. A technical meaning for each item of Table 2 willbe described in detail below.

TABLE 1 Si Ti Fe Al Si/Ti Classification (at %) (at %) (at %) (at %)(Fe) Comparative Example 1 70 15 15 0 4.66 Comparative Example 2 70 14.514.5 1 4.82 Comparative Example 3 60 10 10 20 6 Example 1 70 12.5 12.5 55.6 Example 2 70 11.5 11.5 7 6.08 Example 3 70 10 10 10 7 Example 4 68 99 14 7.5 Example 5 65 10 10 15 6.5 Example 6 70 10.5 10.5 9 6.66 Example7 67 10 10 13 6.7 Example 8 66 9.93 9.93 14.14 6.64 Comparative Example4 70 7.5 7.5 15 9.3 Comparative Example 5 70 6.5 6.5 17 10.76

In the exemplary embodiment of the present invention, the items weremeasured by repeating charge/discharge 50 times. The charge/dischargemethod was performed based on a charge/discharge method for an activematerial for a lithium secondary battery which is generally and publiclyknown in the art.

First, in Examples 1 to 8 of the present invention, Al is composedwithin a range of 5 to 19% based on at %, and Comparative Example 1represents a case where Al is not added, and Comparative Example 2represents the case where Al is added by 1%, and Comparative Example 3represents the case where Al is added by 20%.

In the meantime, Comparative Example 4 represents the case where Al isadded by 15% and Ti (Fe) is added by 7.5%, and Comparative Example 5represents the case where Al is added by 17% and Ti (Fe) is added by6.5%.

In the meantime, Ti and Fe are bonded to Si to form Si₂TiFe that is anintermetallic compound. Accordingly, when the contents of Ti and Fe arelarge, Si is consumed for forming the intermetallic compound, so that acapacity of Si per g of an active material is decreased, and in thiscase, in order to obtain Si with a capacity of 1,000 mAh/g or more, thecontent of Si inserted needs to be considerably increased. In general,when a large amount of Si that is a semimetal is contained, viscosity ofa molten metal is high during melting, and thus rapid solidificationworkability becomes poor, so that the content of Si is preferablymaintained within a range of 70%. Accordingly, it is preferable that thecontents of Ti and Fe do not exceed 14% considering the forming of theintermetallic compound with Si.

In the Examples of the present invention, it was derived that it ispreferable that the contents of Ti or Fe has a range of 9% to 14% duringa process of drawing an optimum alloy component in relation to anexpansion rate.

Referring to Tables 1 and 2, in the Examples of the present invention,it was derived that it is preferable that a ratio of Ti to Fe in thenegative active material for the secondary battery has 1:1 and a ratioof Si:Ti or Si:Fe in the negative active material for the secondarybattery has a range of 5:1 to 9:1.

Further, Al may have a range larger than 1% and less than 20% based onat %. When about 1% of Al is included, expansion of the active materialis severely incurred after 50 cycles, and in this case, the activematerial is dispersed, so that about 1% of Al is not preferable.Further, when 20% of Al is included, a discharge capacity is sharplydecreased by a change in a volume fraction of Si:matrix, so that 20% ofAl is not preferable. In the Examples of the present invention, it wasderived that when Al has a range of 5 to 19% based on at %, the range ofthe most preferable expansion rate is obtained, and it could be seenthat a discharge capacity is not decreased within the range of 5 to 19%.Most preferably, Al is 10 to 19%, and it is possible to obtain the rangeof a most preferable 50 cycle expansion rate, and further a dischargecapacity is not decreased.

Referring to Table 2 below, in Examples 1 to 8 of the present invention,it can be seen that performance of the active material is improvedaccording to the addition of Al. Particularly, it can be seen that whenAl is added, a discharge capacity, reversible efficiency, and anexpansion characteristic are remarkably improved. By contract, inComparative Example 1, in which Al is not added, a 50 cycle expansioncharacteristic has a value exceeding 200%. Further, in ComparativeExample 2 in which 1% of Al is added, a 50 cycle expansioncharacteristic exceeds 200% similar to Comparative Example 1. Bycontrast, in Comparative Example 3 in which 20% of Al is added, a 50cycle expansion characteristic exceeds is 40.2%, which is very low, butin this case, a discharge capacity is remarkably decreased, so thatthere is a problem in that an effect of performance improvement of thenegative active material of a secondary battery is rather decreased.

In the meantime, a ratio of Si/Ti(Fe) in Comparative Example 4 having acomposition of 15% Al and 7.5% Ti(Fe) has 9.3, and a ratio of Si/Ti(Fe)in Comparative Example 5 having a composition of 17% Al and 6.5% Ti(Fe)has 10.76. Therefore, a 50 cycle expansion characteristic is very lowwhen a composition of Ti(Fe) is less than 9% and a ratio of Si/Ti(Fe)exceeds 9.

Accordingly, in the Examples of the present invention, referring toTables 1 and 2, it can be seen that a discharge capacity, reversibleefficiency, and an expansion characteristic of the negative activematerial are remarkably improved according to the addition of Al.

Further, it can be seen that when the content of Al exceeds at least 1%,and is less than 20% based on at %, optimum performance is exhibited.Further, it can be seen that in Comparative Examples 1 and 2, anamorphization degree (%) is less than 25%, and a ratio of Si/Ti(Fe) isalso less than 5. Thus, it can be seen that in the Examples of thepresent invention, a preferable ratio of Si:Ti or Si:Fe in the negativeactive material for the secondary battery has a range of 5:1 to 9:1.

FIGS. 1A, 1B, and 1C and FIG. 2 are tissue pictures showing an expansionrate characteristic after 50 cycles for Comparative Example 2 andExample 5, respectively. In FIGS. 1A, 1B, and 1C, it can be seen that apart forming a bright particle shape is a matrix, and a dark backgroundpart is Si, and the matrixes are well collected at an initial stagebefore a lifespan test similar to FIG. 1C, but bright particles formingthe matrix are dispersed while charge/discharge for 50 cycles isrepeated and a volume of an Si part is increased. As illustrated in FIG.1C, even after 50 cycles, the matrixes are not dispersed from each otherand are collected well regardless of contraction and expansion ofsilicon. A phenomenon, in which the active material matrixes aredispersed, cause a rapid increase in an expansion numerical value after50 cycles. When 1% or less of Al is added similar to ComparativeExamples 1 and 2, an expansion rate after 50 cycles is 200% or more,which is very high, by contrast, in Example 5, in which the dispersionof the active material is not observed, it can be seen that an expansionrate after 50 cycles is about 78%, which is very excellent, and alifespan characteristic is also very excellent.

TABLE 2 1CY- Amorphization 1CY- 1CY- 1CY- Electrode 50CY- 50CY- 50CY-50CY- degree Classification charge discharge efficiency plate dischargeefficiency maintenance expansion (%) Comparative 1134.0 924.2 81.5%800.4 871.2 98.4% 101.6% 210.0% 24.5 Example 1 Comparative 1277.2 1072.383.9% 928.6 1012.1 98.8% 96.1% 208.3% 24.7 Example 2 Comparative 614.2432.8 70.3% 374.8 597.7 100.5% 164.1%  40.2% 46.5 Example 3 Example 11299.9 1085.2 83.5% 939.8 948.7 99.1% 91.8% 147.9% 29.2 Example 2 1405.61212.5 86.3% 1050.0 1125.1 99.5% 97.1%  96.2% 41.1 Example 3 1336.51133.2 84.7% 981.3 1038.7 99.3% 97.2% 120.0% 45.5 Example 4 1752.31535.8 87.6% 1330.0 1216.5 99.3% 88.7%  93.8% 35.1 Example 5 1189.4988.0 83.0% 855.6 977.2 100.5% 113.2%  78.1% 45.3 Example 6 1608.81375.2 85.5% 1245.8 99.4% 98.8%   110% 43.1 Example 7 1278 1062   83%1037 99.5% 97.6%  82.8% 47.7 Example 8 1465 1243 84.8% 1069 99.6% 86.1% 76.5% 46.4 Comparative 1822.7 1582.6 86.8% 1311 98.9% 82.9%   180% 35.9Example 4 Comparative 1892.3 1635.1 86.4% 1236 99.1% 75.6%   183% 37.7Example 5

First, the active material in the Example of the present invention wasevaluated by manufacturing an electrode plate having a compositionbelow.

A silicon alloy active material was evaluated by manufacturing anelectrode plate having a composition in which a ratio of a conductiveaddictive (based on a carbon black) and a binder (based on an organicmaterial, a PAI binder) is 86.6%:3.4%:10%, and slurry dispersed in anNMP solvent was prepared, the slurry was coated on a copper foiledcurrent collector by a doctor blade method, followed by drying in amicrowave oven at 110° C. and heat treating for one hour at an Aratmosphere and 210° C., to cure a binder.

The electrode plate manufactured by the aforementioned method wasassembled to a coin cell by using a lithium metal as a counter electrodeand was subject to the formation process under the condition below.

Charge (lithium insertion): 0.1C, 0.005V, 0.05C cut-off

Discharge (lithium discharge): 0.1C, 1.5V cut-off

After the formation process, a cycle test was performed under thecondition below.

Charge: 0.5C, 0.01V, 0.05C cut-off

Discharge: 0.5C, 1.0V cut-off

In Table 2, 1CY-charge (mAh/g) is a formation charge capacity per 1 g ofan active material, and is a value obtained by measuring a chargequantity at a charge stage in the formation process that is the firstcharge stage after assembling the coin cell and dividing the measuredcharge quantity by weight of the active material included in the coincell electrode plate.

1CY-discharge (mAh/g) is a formation discharge capacity per 1 g of theactive material, and is a value obtained by measuring a charge quantityat a discharge stage in the formation process that is the first chargingstage after assembling the coin cell and dividing the measured chargequantity by weight of the active material included in the coin cellelectrode plate. In the present Example, a capacity per g means a 0.1Cdeformation discharge capacity that is the discharge capacity measuredin this case.

1CY-efficiency is a value, expressed by a percentage, obtained bydividing a discharge capacity in the formation process that is the firstcharge/discharge process by a charge capacity.

In general, graphite has high initial efficiency of 94%, a silicon alloyhas initial efficiency of 80 to 90%, and a silicon oxide (SiOx) hasinitial efficiency of a maximum of 70%.

Any kind of material has initial efficiency of less than 100% becauselithium initially injected while charging during the formation processis irreversibly trapped or consumed by a side reaction, such asformation of an SEI, and when initial efficiency is low, there is a lossin that a negative active material and a positive active material needto be additionally injected, so that it is important to improve initialefficiency when designing a battery.

The silicon alloy used in the Example of the present invention has aninitial efficiency value of 85%, and the conductive addictive or thebinder initially and irreversibly consumes lithium, so that an initialefficiency value of the active material itself is substantially about90%.

50CY-discharge is a discharge capacity per g of the active material for50 cycles, and is a value obtained by dividing a charge quantitymeasured during the discharge at the 50^(th) cycle including theformation process during the cycle test progressed with 0.5C after theformation process by weight of the active material. When the activematerial deteriorates during the progress of the cycle test,50CY-discharge is represented by a numerical value smaller than aninitial discharge capacity, and when the active material hardlydeteriorates during the progress of the cycle test, 50CY-discharge isrepresented by a numerical value similar to an initial dischargecapacity.

50CY-efficiency is a ratio, expressed by a percentage, of a dischargequantity to a charge quantity at the 50 cycle. High 50CY-efficiencymeans that a loss of lithium by a side reaction and other deteriorationat a corresponding cycle is small. In general, when the 50CY-efficiencyis 99.5% or more, the value is determined as a very excellent value, anddistribution in the assembling of the coin cell cannot be ignorableaccording to an environment of an experiment room, so that even when the50CY-efficiency is 98% or more, the value is determined as an excellentvalue.

50CY-maintenance is a ratio, which is expressed by a percentage, of adischarge capacity at the 50^(th) cycle based on a discharge capacity atthe first cycle when a next 0.5C cycle is performed except for the cycleperformed during the formation process.

When the 50CY-maintenance rate is large, it is considered that aninclination of a battery lifespan close to a horizontal line, and whenthe 50CY-maintenance rate is 90% or less, it means that deterioration isincurred during the progress of the cycle and a discharge capacity isdecreased. In some Examples, there are even the cases where the50CY-maintenance ratio exceeds 100%, and this is determined asdeterioration is hardly incurred for a lifespan, and activated siliconparticles are additionally present.

50CY-expansion is a thickness increased value, which is expressed by apercentage, after 50 cycles compared to an initial electrode platethickness. A method of measuring the 50CY-expansion will be described indetail below.

First, an initial thickness of a current collector is measured.

Then, a thickness of only the active material is calculated by measuringa thickness of an electrode plate, which is cut in a circular shape soas to be assembled with the coin cell, by using a micro meter, and thensubtracting the thickness of the current collector from the measuredthickness of the electrode plate.

Next, after a 50 cycle test is completed, the coil cell is removed froma dry room, only a negative electrode plate is separated, an electrolyteleft on the electrode plate is washed by using a DEC solution and dried,a thickness of the electrode plate is measured by using a micro meter,and a thickness of the current collector is subtracted from the measuredthickness of the electrode plate to calculate a thickness of only theactive material after the cycle. That is, a value, which is expressed bya percentage, obtained by dividing an increased thickness of the activematerial after 50 cycles compared to an initial thickness of the activematerial by the initial thickness of the active material is50CY-expansion.

It will be appreciated by those skilled in the art that the presentinvention described above may be implemented into other specific formswithout departing from the technical spirit thereof or essentialcharacteristics. Thus, it is to be appreciated that embodimentsdescribed above are intended to be illustrative in every sense, and notrestrictive. The scope of the present invention is represented by theclaims to be described below rather than the detailed description, andit should be interpreted that all the changes or modified forms, whichare derived from the meaning of the scope of the claims, the scope ofthe claims, and the equivalents thereto, are included in the scope ofthe present invention.

The invention claimed is:
 1. A negative active material for a secondarybattery, which is an alloy formed by a chemical formula below, and inwhich a ratio of Ti to Fe in the negative active material for thesecondary battery has 1:1 and a ratio, R, of Si:Ti or Si:Fe in thenegative active material for the secondary battery has ranges of5.6:1≤R<7:1 and 7:1<R≤8.55:1, wherein an amorphization degree of a finecrystalline region on a matrix within the alloy is 29.2˜47.7%, whereinan expansion rate of the negative active material after 50 cycles has arange of 70 to 150%, and wherein Si content is greater than 70 at % andless than 77 at %, and wherein the amorphization degree is expressed as:amorphization degree (%)=((entire area−crystallization area)÷entirearea), and wherein the chemical formula is expressed as:Si_(x)Ti_(y)Fe_(z)Al_(u) (x, y, z, and u are at %, x: 1−(y+z+u), y: 0.09to 0.125, z: 0.09 to 0.125, and u: 0.05 to 0.15).
 2. The negative activematerial of claim 1, wherein a discharge capacity of the negative activematerial for the secondary battery after 50 cycles is 90% or morecompared to an initial discharge capacity.
 3. The negative activematerial of claim 1, wherein efficiency of the negative active materialfor the secondary battery after 50 cycles is 98% or more.
 4. A secondarybattery, comprising: a negative electrode including a negative activematerial, wherein the negative active material is an alloy formed by achemical formula below, and a ratio of Ti to Fe in the negative activematerial for the secondary battery has 1:1, and a ratio, R, of Si:Ti orSi:Fe in the negative active material for the secondary battery hasranges of 5.6:1≤R<7:1 and 7:1<R≤8.55:1, wherein an amorphization degreeof a fine crystalline region on a matrix within the alloy is 29.2˜47.7%,wherein an expansion rate of the negative active material after 50cycles has a range of 70 to 150%, and wherein Si content is greater than70 at % and less than 77 at %; a positive electrode; and an electrolyte;wherein the amorphization degree is expressed as:amorphization degree (%)=((entire area−crystallization area)÷entirearea), and wherein the chemical formula is expressed as:Si_(x)Ti_(y)Fe_(z)Al_(u) (x, y, z, and u are at %, x: 1−(y+z+u), y: 0.09to 0.125, z: 0.09 to 0.125, and u: 0.05 to 0.15).
 5. The secondarybattery of claim 4, wherein a discharge capacity of the negative activematerial for the secondary battery after 50 cycles is 90% or morecompared to an initial discharge capacity.
 6. The secondary battery ofclaim 4, wherein efficiency of the negative active material for thesecondary battery after 50 cycles is 98% or more.